1. Field of the Invention
The present invention relates to wireless communication systems and, in particular, to a carrier frequency estimation method and apparatus in an Orthogonal Frequency Division Multiplexing (OFDM) communication system.
2. Description of the Related Art
Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmission technique whose history dates back to the mid-1960's and which has been in the limelight with the development of Very Large Scale Integration (VLSI). In an OFDM system, the system frequency band is divided into a number of subcarriers carrying data in parallel and it can be simply implemented with Fast Fourier Transform (FFT). Recently, OFDM has been adopted in various communication systems due to its superiority in spectral efficiency as compared to single carrier modulation schemes.
The OFDM scheme is robust to frequency selective fading channels as compared to the single carrier modulation scheme. This is because the frequency selective channel in the system frequency band composed of a plurality of subcarriers is reduced into a frequency nonselective channel in the subcarrier band that can be compensated through a simple equalization process. Particularly in OFDM, a cyclic prefix (which is a copy of the last part of the OFDM symbol) is inserted in front of each OFDM symbol so as to avoid Inter-Symbol Interference (ISI). With these beneficial characteristics, such as robustness to the frequency selective fading channel and ISI avoidance, OFDM has been chosen as the multiplexing method in broadband communication systems.
In the OFDM system, a carrier frequency synchronization algorithm is used to compensate the carrier frequency offset between the transmitter and receiver. The carrier frequency offset is present due to the difference between the reference frequencies of the local oscillators at the transmitter and receiver and the Doppler Effect of the channel. The carrier frequency offset of the input signal at the receiver can be greater than the interval between subcarriers. A process to compensate the integer part of the subcarrier frequency offset is defined as “coarse carrier frequency synchronization”, while a process to compensate the fractional part of the subcarrier frequency offset is defined as “fine carrier frequency synchronization”. Since the integer part of carrier frequency offset shifts the OFDM signal as much as the integer multiple in frequency domain, it has the effect of shifting the FFT output sequence.
In the meantime, the fractional part of the subcarrier frequency offset causes interference between the FFT outputs, resulting in a significant Bit Error Rate (BER). Typically, it is known that the OFDM system is more vulnerable to the carrier frequency offset as compared to the single carrier transmission system. In the present invention, a method for compensating a fine carrier frequency offset is provided.
The conventional carrier frequency offset estimation methods can be classified into two types: blind frequency offset estimation in time domain, and training symbol-based frequency offset estimation in frequency domain.
The blind frequency offset estimation method uses the Cyclic Prefix (CP) which is a copy of the last part of the OFDM symbol, which is inserted in front of the OFDM symbol. That is, the blind frequency offset estimation method uses the fact that the variation of the correlation between the CP and the last part of the OFDM symbol corresponds to the frequency offset. Although widely used, this method has drawbacks in that the frequency offset estimation performance is deteriorated as a number of signal paths increases, and especially when DC offset exists in the CP, the DC offset causes bias to the frequency offset estimation value.
The training symbol-based frequency offset estimation method uses the fact that the variation of two consecutive OFDM symbols corresponds to the frequency offset under the assumption that the two consecutive OFDM symbols are in an identical pattern.
The blind frequency offset estimation method is a time domain processing method, and the frequency offset estimation method using consecutive training symbols is a frequency domain processing method. Accordingly, in the frequency domain processing method, the estimation performance depends on the channel characteristics.
In the cellular environment, in which a mobile station receives signals from multiple base stations, the frequency offset values of the received signals from the respective base stations are likely to vary while the mobile station is on the move. Particularly when the mobile station is moving fast at the cell edge, the frequency offset variation occurs frequently, whereby the demodulation performance to the serving cell is deteriorated by the frequency offsets of the signals received from other neighbor base stations.
Among the conventional frequency synchronization methods for use in the OFDM receiver, a time domain estimation method using a CP in a single OFDM symbol estimates the frequency offset using all the signals received from serving and neighbor base stations of the cellular system. Accordingly, the signal received from the serving cell (i.e. serving base station) is influenced by the signals received from other neighbor base stations, thereby increasing the frequency offset and thus deteriorating the BER performance. In the meantime, the conventional frequency domain estimation method using two consecutive OFDM symbols estimates the frequency offset by calculating the correlation between the two consecutive OFDM symbols at the same subcarrier under the assumption that the two consecutive OFDM symbols are identical with each other. In the Long Term Evolution (LTE) system, however, the assumption of two consecutive OFDM symbols having an identical symbol pattern is not applied, and thus the conventional frequency domain frequency offset estimation method cannot be adopted.